1. Field of the Invention
This invention relates to bonding tool tips in general and more particularly to ceramic tool tips for bonding electrical connections.
2. Description of the Prior Art
Integrated circuits are typically attached to a lead frame, and individual leads are connected with wire to individual bond pads on the integrated circuit. The wire is fed through a tubular bonding tool tip having a bonding pad at the output end. These tips are called capillary tips. An electrical discharge at the bonding tool tip supplied by a separate Electronic Flame Off (EFO) device melts a bit of the wire, forming a bonding ball. Other bonding tools do not have the center tube, but have a feed hole or other feature for feeding the wire along, as needed. Some bonding tool tips have no such arrangement for feeding wire, such as bonding tool tips for magnetic disk recording devices, where the wire is insulated and bonded to a magnetic head and then to a flexible wire circuit.
When the bonding tool tip is on the integrated circuit die side of the wire connection, the wire will have a ball formed on the end of the wire, as above, before reaching the next die bonding pad. The ball then contacts the film formed on the die pad on the integrated circuit. The bonding tool tip is then moved from the integrated circuit die pad, feeding out gold wire as the tool is moved, onto the bond pad on the lead frame, and then scrubbed laterally by an ultrasonic transducer. Pressure from the bonding tool tip and the transducer, and capillary action, causes the wire to “flow” onto the bonding pad where molecular bonds produce a reliable electrical and mechanical connection.
Bonding tool tips must be sufficiently hard to prevent deformation under pressure, and mechanically durable so that many bonds can be made before replacement. Prior art bonding tool tips were made of aluminum oxide, which is an insulator that is durable enough to form thousands of bonding connections. Bonding tool tips must also be designed to produce a reliable electrical contact, yet prevent electrostatic discharge damage to the part being bonded. Certain prior art devices emit one or more volts when the tip makes bonding contact. This could present a problem, as a one volt static discharge could cause a 20 milliamp current to flow, which, in certain instances, could damage the integrated circuit or magnetic recording head.
U.S. Pat. No. 5,816,472 to Linn describes a durable alumina bonding tool “without electrically conductive metallic binders” that is therefore an insulator. U.S. Pat. No. 5,616,257 to Harada describes covering a bonding tool electrode with an insulating cap or covering “made of a ceramic material” to produce a large electrostatic discharge that creates bonding balls of stable diameter. U.S. Pat. No. 5,280,979 to Poli describes a vacuum wafer-handling tool having a ceramic coating “made with a controlled conductivity” to prevent a large electrostatic discharge.